media: v4l2-core: add v4l2_debugfs_root()

This new function returns the dentry of the top-level debugfs "v4l2"
directory. If it does not exist yet, then it is created first.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>